Electric field crowding can play a significant role in limiting performance of semiconductor switching devices, particularly when such devices are used in high power applications and are required to withstand large blocking voltages. FIG. 1 illustrates a lateral field effect transistor having an N+ source region 14 formed within a P-type well region 8, an N+ drain 12 region formed in an N-type substrate 2, an insulated gate electrode 10 and a gate insulator 6. A field oxide isolation region 4 is also provided and contact is made to the P-type, well region by a P+ contact region 16 within the well region 8. As will be understood by those skilled in the art, electric field crowding in the region extending between the drain region 12 and the gate electrode 10 (region 18) may limit the withstand voltage of the transistor of FIG. 1.
FIG. 2 illustrates an attempt to modify the structure of the transistor of FIG. 1 to reduce field crowding problems. In particular, FIG. 2 illustrates a lateral field effect transistor having an N+ source region 32 formed within a P-type well region 26, an N+ drain region 28 formed in an N-type substrate 20, a gate insulator comprising portions of insulating region 24a and field oxide isolation region 22 and a gate electrode 30 formed on the gate insulator. Contact is made to the P-type well region 26 by a P+ contact region 34. The transistor of FIG. 2 has improved withstand voltage compared to the device of FIG. 1 because the drain region 28 is formed at remote location (at an opposite side of the field oxide isolation region 22 relative to the source 32), however, the active channel region 36 is relatively long which means the on-state resistance of the transistor of FIG. 2 may be relatively large and the on-state current drive capability may be relatively low.
Thus, notwithstanding attempts to form switching devices having high withstand voltage capability, there continues to be a need for improved switching devices which are less susceptible to field crowding, but still have favorable on-state characteristics.